By somebody poking their finger into the shutter? I would be inclined to give it a very thorough visual and technical examination (Nikon, like Canon, has diagnostic devices that plug into the camera to read-off various statistics that can be quite revealing and quite contradictory to the oh-so-sweet salesman's pitch. Go for it.
That could be the case here. I don't know the man/woman who has left the camera to the shop. The shop gives me full waranty so I don't mind not knowing. I suppose it's send over to a camera repair point where everything will be checked with the above mentioned devices. Just looking at it is what I already did. I didn't "see" or "heard" a problem... Thanks for your input here.